Method and apparatus to measure self-capacitance using a single pin

Abstract

A method for measuring capacitance in a sensor device using an internal reference circuit element(s), and without implementing additional circuitry and devices external to the sensor device, is described. In some embodiments a method uses an output pin of the sensor device and an internal reference capacitor of the sensor device to identify a touch applied to a touch point or electrode coupled to the touch sensor. The method applies reference voltages to charge the reference capacitor and measure a signal received from an electrode, wherein the touch sensor controls switching within the touch sensor to apply the reference voltages to the reference capacitor.

The invention claimed is: 
     
       1. A touch sensor system, comprising:
 at least one input port to receive an input signal from at least one electrode, the at least one electrode having a capacitance; 
 an Analog to Digital Converter (ADC) having an ADC input and an ADC output; 
 a capacitor coupled to the ADC input; 
 a multiplexer (MUX) having multiple MUX inputs and a MUX output that is coupled to the ADC input, wherein the at least one input port is coupled to one of the multiple MUX inputs; 
 a first switch for coupling a first reference voltage to a first input of the multiple MUX inputs; 
 a second switch for coupling a second reference voltage to the first input of the multiple MUX inputs; 
 a switch controller operable to:
 control the first switch to apply the first reference voltage to the first input of the MUX; and 
 control the second switch to apply the second reference voltage to the first input of the MUX; and 
 
 a sensor controller operable to:
 receive a first digital value from the ADC output after application of the first reference voltage; 
 receive a second digital value from the ADC output after application of the second reference voltage; 
 determine, based at least upon the first and second digital values, an amount of mains interference; 
 remove the determined amount of mains interference from the first digital value; and 
 determine a change in the capacitance of the at least one electrode as a function of the first digital value. 
 
 
     
     
       2. The touch sensor system of  claim 1 , further comprising:
 a MUX controller operable to select the at least one electrode as an input to the ADC input; and 
 wherein the sensor controller is further operable to determine a change in the capacitance of the at least one electrode as a function of the first digital value and the second digital value. 
 
     
     
       3. The touch sensor system of  claim 1 , wherein the first reference voltage is opposite in polarity to the second reference voltage. 
     
     
       4. The touch sensor system of  claim 1 , wherein the sensor controller is further configured to evaluate the first and second digital values against a threshold value to detect a touch at the at least one electrode. 
     
     
       5. A method, comprising:
 controlling a first switch to charge a reference capacitor to a first voltage level, the reference capacitor coupled to an input of an Analog to Digital Converter (ADC); 
 receiving a first analog signal from a touch sensor electrode at the input of the ADC; 
 converting the first analog signal to a first digital value provided at an output of the ADC; 
 controlling a second switch to charge the reference capacitor to a second voltage level by applying a second voltage; 
 receiving a second analog signal from the touch sensor electrode at the input to the ADC; 
 converting the second analog signal to a second digital value at the output of the ADC; 
 determining, based at least upon the first and second digital values, an amount of mains interference; 
 removing the determined amount of mains interference from the first digital value; 
 detecting a touch to the touch sensor device based on the first digital value. 
 
     
     
       6. The method of  claim 5 , further comprising
 controlling a multiplexer (MUX) to select an input to the ADC. 
 
     
     
       7. The method of  claim 5 , wherein the reference capacitor is further coupled to a reference voltage, and the first voltage level is opposite in polarity to the second voltage level with respect to the reference voltage. 
     
     
       8. The method of  claim 7 , wherein the reference voltage is a function of the first voltage level. 
     
     
       9. The method of  claim 5 , wherein the second voltage level is a ground voltage. 
     
     
       10. The method of  claim 5 , wherein detecting a touch further comprises:
 calculating a difference between the first digital value and second digital value; and 
 comparing the difference to a threshold value. 
 
     
     
       11. The method of  claim 5 , wherein controlling the first switch to charge the reference capacitor to the first voltage level comprises closing the first switch and opening the second switch. 
     
     
       12. The method of  claim 5 , wherein controlling the second switch to charge the reference capacitor to the second voltage level comprises closing the second switch and opening the first switch. 
     
     
       13. The method of  claim 5 , wherein converting the first analog signal to a first digital value at the output of the ADC comprises opening the first and second switches, and wherein converting the second analog signal to a second digital value at the output of the ADC comprises opening the first and second switches. 
     
     
       14. A non-transitory computer-readable medium comprising instructions which, when implemented by one or more machines, cause the one or more machines to:
 control a first switch to charge a reference capacitor to a first voltage level, the reference capacitor coupled to an input of an Analog to Digital Converter (ADC); 
 receive a first analog signal from a touch sensor electrode at the input of the ADC; 
 convert the first analog signal to a first digital value provided at an output of the ADC; 
 control a second switch to charge the reference capacitor to a second voltage level; 
 receive a second analog signal from the touch sensor electrode at the input to the ADC; 
 convert the second analog signal to a second digital value at the output of the ADC; 
 determine, based at least upon the first and second digital values, an amount of mains interference; 
 remove the determined amount of mains interference from the first digital value; 
 detect a touch to the touch sensor device based on the first and second digital values. 
 
     
     
       15. The non-transitory computer-readable medium of  claim 14 , wherein to detect a touch is further configured to:
 calculate a difference between the digital value and second digital value; and 
 compare the difference to a threshold value. 
 
     
     
       16. The non-transitory computer-readable medium of  claim 14 , wherein:
 to convert the first analog voltage to the first digital value at the output of the ADC is further to open the first and second switches; and 
 to convert the second analog voltage to the second digital value at the output of the ADC is further to open the first and second switches.